Nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries are known to have a high energy density. Owing to this, nonaqueous electrolyte secondary batteries have been typically used as power sources for portable electronic equipment. Moreover, applications of nonaqueous electrolyte secondary batteries have recently been expanded to the energy sources for hybrid transport equipment (e.g., hybrid cars, hybrid bicycles) or electromotive transport equipment (e.g., electric vehicles, electric motorcycles).
Additionally, use of nonaqueous electrolyte secondary batteries as batteries for large-scale power storage is also being fully studied.
Still, the energy density of the nonaqueous electrolyte secondary batteries currently in practical use is, when evaluated in terms of, e.g., maximum continuous Internet connection time of smartphones or maximum travel distance per one charge of electric vehicles, not considered to be sufficiently high to meet such needs. As such, research and development for improving the energy density of nonaqueous electrolyte secondary batteries are being conducted. More concretely, as the improvement in a capacity density of active materials may lead to a drastic improvement in the energy density of nonaqueous electrolyte secondary batteries, attempts are being made to apply various materials to cathode active materials and anode active materials.
For example, graphite, carbon-based materials and lithium titanium oxide (LTO) are known as anode active materials. Further, silicon (Si) has a greater capacity than these anode active materials and the capacity of Si alone is about 10 times that of graphite. The Li absorption mechanism of Si as an anode active material corresponds to an alloying reaction to form Li—Si alloy, and involves a very large volume change. Thus, use of Si alone as an anode material will accelerate size reduction of the active material particles and collapse of the electrode due to the volume change at the time of charge and discharge of the battery, resulting in early deterioration of the battery capacity. Accordingly, in using Si as an anode material, the influence of the volume change is suppressed typically by altering Si into SiO through partial oxidation, by adopting Si in the form of composite active material particles such as a coating, by compounding Si with a carbon-based material within electrode active material layers, or the like.
Use of such composite Si may realize a lithium ion secondary battery having a practical level of cycle life as well as a large energy density. Nevertheless, batteries employing composite Si as an anode involve a large volume change of Si, which easily causes a break of an electrode (mode concretely, anode) and particles and the side-reactions due to increase of their surface areas, as compared to the conventional batteries employing graphite as an anode material. As such, in the batteries employing composite Si as an anode, deteriorating reactions of the anode (specifically, capacity decrease of the anode and a reaction to consume Li stored in the anode) progress more rapidly than in the conventional batteries employing graphite as an anode material.
Moreover, if a cathode hardly deteriorates while the deteriorating reactions of an anode rapidly progress, the capacity ratio between the cathode and the anode, as well as the SOC (State Of Charge) position will change. Charging the battery having such a changed internal state under the same conditions as prior to the change (e.g., at the start of use) would promote the deterioration of the battery capacity.
More concretely, where a charge and discharge potential curve shows a gentle gradient as with the anode using graphite or LTO, even a change in the SOC position of the anode will not largely influence the deteriorating speed of the battery capacity. On the other hand, when a battery combines an anode providing a charge and discharge potential curve of a steep gradient (e.g., Si that is compounded or mixed with a carbon material) and a cathode having a capacity in the high potential region, a shift of the SOC position of the anode will largely change the end-of-charge potential of the cathode, resulting in the cathode being charged in the high potential region that is not normally used as a charge and discharge capacity and therefore in fast deterioration.